I need to use 50 uM concentrations. I calculated it in two ways and (sic!) I got 2 completely different results, could anybody help with that?

Here are the details of my calculations:
I. Assuming that I got exactly what the company claimed to send so 1mg.
II. Dissolving the whole amount which I received in 1ml and measuring the absorbance at 495 nm with the spectrophotometer